High poly mesh slow in blender?
You may retopologize your model and then bake normals from the high poly model to the low poly one. This will save all the details but the geometry won't be as dense, so it will be more animation friendly.


First you have to retopologize your high poly model (take a look at these answers: How to easily make mesh retopology? and How to convert from high poly to low poly).
There are two models pictured below. The high poly one with a very dense geometry on the left and low poly one with a simple retopologized geometry on the right. With your retopologized model you'll be able to bake the details from the high poly model to the low poly one. 


Here's how to do it. First unwrap your low poly mesh and create a new texture with the desired parameters. 


Next select (with the RMB) the high poly model, then select the low poly one.


Go to Render data header and under Bake panel change the mode to Normals, check the Selected to Active checkbox, then press the Bake button. After Blender finishes baking save the image to your computer.


Now give your low poly model a material, go to Texture header, open your previously saved image and set the settings as pictured below.


To see the normal map in action change the viewport shading type to Rendered and set the GLSL option in the Shading panel of Properties Shelf (N).